1. Field of the Invention
The present invention relates to a porous body for a solid electrolyte capacitor. More particularly, the present invention relates to a porous body for a solid electrolyte capacitor which consists of a titanium-aluminum (Ti-Al) alloy which has a double rugged structure and to a process for producing the same.
As used herein, the term "double rugged structure" is such a porous structure which consists of large voids producing a knobby, rough, hill and vale texture on the surface of primary particles.
2. Description of the Prior Art
At present, tantalum has been widely used as a valve metal for a solid electrolytic capacitor. However, despite its excellent properties, a tantalum (Ta) solid electrolytic capacitor has no great promise in the future, from the viewpoint of tantalum resources. In recent years, a large amount of tantalum has been consumed as a material for a capacitor. As a result of this, tantalum-containing ore, which in itself is a rare resource, has been increasingly exhausted. The exhaustion of the tantalum resources results in a substantial increase of cost of tantalum, which in turn will lead to a loss of marketability for the tantalum capacitor in the near future because of its inevitably increasing cost. Accordingly, there is a strong need for a solid electrolytic capacitor made of a material capable of substituting for tantalum. As an anode material for a solid electrolytic capacitor, such material should meet the following requirements:
(1) A material cost is cheap; PA1 (2) It can be formed into a porous body having a high surface area; PA1 (3) A capacitance per unit area (.mu.FV/cm.sup.2) is high; and PA1 (4) Properties, other than capacitance, such as dielectric loss and leakage current, are equivalent to or better than the corresponding properties of the Ta solid electrolytic capacitor.
One anode material for a solid electrolytic capacitor, which can substantially satisfy the above-mentioned requirements, is described in U.S. Pat. No. 3,599,053. An Al-Ti alloy porous body is made by vacuum melting or arc melting of Al-Ti alloy powder. However, the vacuum melting or arc melting process has the disadvantages since the production process for the porous body is complicated, and since the resultant porous body has a low specific surface area which makes it difficult to produce a capacitor having excellent properties.
On the other hand, the inventors of the present invention have described a process for producing a Ti-Al porous body, in U.S. patent application Ser. No. 82,217. This process comprises press-shaping a mixture of finely divided titanium and aluminum powder and then subjecting the press-shaped body to a suitable heat treatment. In accordance with this process, it is easy to obtain a porous body of Al-Ti alloy having a density of 20 to 70% of a theoretical density and a specific surface area from 0.01 to 1.0 m.sup.2 /g. In this process, a porous body of Al-Ti alloy having a high specific surface area can be made directly from Ti powder and Al powder, and therefore, the production process is simple. The wet properties of the porous body indicated that an excellent high capacitance capacitor exhibiting a low leakage current and dielectric loss can be made therefrom.
However, the Ti-Al porous body, and the process therefor, according to U.S. patent application Ser. No. 82,217 (now U.S. Pat. No. 4,331,477) suffer from the following disadvantages. That is, in order to obtain a porous body having a high specific surface area which may be a replacement for a porous body made of tantalum, it is necessary to use a super-fine powder, with an average grain having a diameter of about several microns. Although such a fine powder can provide a porous body having a desired specific surface area, it is naturally excessively oxidized. Therefore, the resultant porous body is remarkably inferior in respect of the leakage current and dielectric loss when it is formed into a solid electrolytic capacitor. Therefore, such a porous body is useless as an anode material for the solid electrolytic capacitor.
On the other hand, in the formation of a cathode of a manganese dioxide layer, a porous Ti-Al alloy body presents another problem when it is obtained merely by remarkably decreasing a particle size while maintaining the conventinal structure thereof. That is, it is difficult and complicated to form a manganese dioxide cathode layer by decomposition of manganese nitrate on a porous anode body having a too fine structure.
There is no other known excellent process for forming a solid cathode which is comparable with the use of manganese nitrate. Accordingly, the Ti-Al alloy porous anode body is also required to have a high resistance to the thermal decomposition treatment of manganese nitrate. Usually, the cathode formation is conducted through a plurality (normally, 5 to 10 times) of thermal decompositions of manganese nitrate.
In the case of the Ta solid electrolytic capacitor, it is well known that the leakage current and dielectric loss after solidification can be greatly varied by changing the thermal decomposition conditions such as temperature-rising speed, atmosphere and the like. Usually, the optimal conditions for these properties have been selected. However, even in the light of these experiences, it is apparent that the leakage current tends to increase as the number of decompositions is increased.
In the Ta solid electrolytic capacitor, it is general that the porous body should be subjected to a re-formation treatment to repair the damaged portion thereof. Also, when the porous body has a conventional structure composed of finely divided particles, each in the form of a sphere, if the size of the particles is micronized, the void structure in the porous body becomes necessarily fine, which prevents a solution of manganese nitrite from penetrating into the interior of the porous body. When such impregnated porous body is subjected to a heat treatment for the thermal decomposition of the manganese nitrate, the absolute amount of manganese dioxide deposited within the porous body is insufficient, which results in an increase in series resistance as a cathode.
As a result of examinations made by the inventors, it was found that an oxide film obtained by subjecting a homogeneous Ti-Al alloy to a formation treatment deteriorates much more remarkably on thermal decomposition of manganese nitrate than it deteriorates in the Ta porous body. It is thought that this deterioration occurs because both Ti and Al essentially are far more chemically active than Ta. A reduction in the number of times that the manganese nitrate decomposition occurs is most effective for preventing the deterioration, i.e. the increase in the leakage current, occurred during the decomposition procedure.
In order to avoid the above-mentioned disadvantages, to reduce the series resistance of the cathode, and the dielectric loss and to realize satisfactory capacitor properties, the voids within the porous body should be large-sized and the specific surface area of the porous body should also be large. The large-sized voids enables a satisfactory cathode formation, even if the number of times of the thermal decomposition of manganese nitrate is reduced. In contrast, with small-sized voids, the cathode should be gradually formed to a desired thickness while preventing a liquid passageway through the void from becoming clogged at the initial stage of the decomposition, by a well known method of repeating the decompositions using a methanol-diluted solution of manganese nitrate. Accordingly, the number of times that the decomposition occurs is inevitably increased.
When a Ti-Al alloy capacitor is to be produced with a capacitance (capacitance/volume) comparable to that of the Ta capacitor, merely by micronizing the structure of a porous body of Ti-Al alloy while maintaining the conventional configuration of the porous body structure, as already described above, the preparation of the porous body is difficult and complicated and the electric properties of the resultant porous body having a manganese dioxide cathode are inferior. Accordingly, it is difficult to replace Ta with a Ti-Al alloy, which is an inexpensive material for industrial purposes.
As described above, the porous body according to U.S. patent application Ser. No. 82,217 had the disadvantage that despite its high specific surface area, the unique characteristic could not be effectively utilized in the production of a solid electrolytic capacitor where the specific surface area is too high.